Method and apparatus for pumping of transition metal ion containing solid state lasers using diode laser sources

ABSTRACT

The present invention encompasses an apparatus for pumping a laser (vibronic or otherwise), which comprises: a transition-metal ion-containing solid state laser gain medium; a means for exciting said laser medium to emit coherent radiation said exciting means being a pumping source comprising at least one laser diode operating at a wavelength shorter than 800 nm; and an optical resonator means for generating and controlling said coherent radiation. 
     The present invention also encompasses a method of pumping a laser (vibronic or otherwise) comprising the steps of: generating a laser diode pumping beam at a wavelength shorter than 800 nm; exciting a transition-metal ion-containing solid state laser gain medium by impinging said laser diode pumping beam on said transition-metal ion-containing solid state laser gain medium, so as to excite the laser medium; and an optical resonator to emit coherent radiation.

This application is a continuation-in-part of application Ser. No.07/640,653 filed on Jan. 14, 1991, now U.S. Pat. No. 5,488,626.

BACKGROUND OF THE INVENTION

This invention relates to methods of and apparatus for the pumping oftransition metal ion containing solid state lasers using visible diodelaser sources operating in the visible spectrum.

Solid state lasers employ dopant ions incorporated in diluteconcentrations in solid hosts as the laser-active gain media. Broadlytunable solid state lasers derive their tunability from emission ofvibrational quanta (phonons) concurrent with the emissions of lightquanta (photons). The energies of the photons and phonons which areemitted simultaneously in a vibronic laser add up to the energy of theassociated purely electronic or"nophonon" transition. The broadwavelength tunability of such a"vibronic laser" derives from the broadenergy phonon continuum which complements the photon emission.

The use of a transition metal ion as a dopant in a solid state lasermedium is known in the art. Walling et at (U.S. Pat. No. 4,272,733)discloses the use of alexandrite, a chromium--doped beryllium aluminate(Cr⁺³ :Be 25 Al₂ O₄) crystal, as a laser medium. The disclosure ofWalling et al U.S. Pat. No. 4,272,733 is incorporated herein byreference.

Alexandrite has been optically pumped with flashlamps in pulsedoperation (See Tunable Alexandrite Lasers, IEEE, J. of QuantumElectronics, Vol. QE-16, No. 12, December 1980, pp. 1302) and arc-lampsin continuous wave ("CW") operation. (See Tunable CW Alexandrite Laser,IEEE, J. of Quantum Electronics, Vol. QE-16, No. 2, February 1980, pp.120). Such flashlamps and arc-lamps have broad band emissions rangingfrom ultraviolet (300 nm) to infrared (1000 nary). Alexandrite, however,predominantly absorbs in the visible wavelength region, approximately400-700 nm. The overlapping of alexandrite's absorption spectrum by theoutput of such lamps results in good but, not ideal, pumpingefficiencies and in substantial heating and consequential thermo opticaleffects.

The pumping of Neodymium ion (Nd⁺³) solid state lasers by,semiconductorlaser diodes had been demonstrated in the 1070's (see W. Koechner,"SolidState Laser Engineering", Springer series in Optical Sciences, vol. 1,chaps. 6, Springer-Verlag, N.Y., 1976). However, it was not thought tobe a practical means for excitation because of limitations on the laserdiodes. In the mid 1980's, the advances in semiconductor diodes lasersimproved their power and reliability, and several to many milliwattdiode lasers became routinely available. Initially these higher powerdiode lasers were Gallium Arsenide GaAs compositions emitting atwavelengths longer than lum. Their development was driven by thecommunications industry interested in 1.3-1.5 μm fiber opticcommunications networks. As the semiconductor growth techniques,primarily molecular beam epitaxy (MBE) and metal organic chemical vapordeposition (MOCVD), matured it became possible to grow and fabricateother III-IV composition laser diodes, notably the ternary compositionsof AlGaAs, lasing at wavelengths as short as 700 nm, but with power andlifetime optimized at wavelengths longer than 750 nm. In 1984 and 1985diode lasers were used to pump Nd:YAG (ytterium aluminum garnet) lasersin laboratory demonstrations of practical devices (see R. Scheps,"Efficient laser diode pumped Nd lasers", Applied optics, Vol. 28, No.1, Jan. 1, 1989, pp. 8-9). Pumping wavelengths were near 820 nm wherethe Nd⁺³ absorption bands were well matched to the highest power outputwavelengths of the AlGaAs diodes.

Prior to this invention, it was believed that diode pumping oftransition metal ion-containing laser materials in general and tunablevibronic laser materials in particular would be impractical for tworeasons: 1) the absorption bands are much broader in the transitionmetal ion-containing laser materials than in the rare earth ions likeNd⁺³ and the absorption strength is weaker unless pumping is done atshort (visible or near visible) wavelengths (typically 700 nm andshorter) and 2) the emission cross section of tunable vibronic lasermedia is typically substantially lower than Nd⁺³ media because theoscillator strength is by necessity spread out over the tuning rangerather than localized to a specific wavelength or few wavelengths. Thismeans that the laser gain is substantially lower for a given excitationlevel and the laser threshold therefore substantially higher for a givenloss.

SUMMARY OF INVENTION

In accordance with the invention, it has been surprisingly found that asolid state laser utilizing a single crystal doped with a transitionmetal ion can be pumped by an exciting means comprising visible diodelasers. The exciting means comprising a diode laser pump beam has awavelength falling within the absorption spectrum of the transitionmetal ion dopant.

The present invention encompasses an apparatus for pumping a laser(vibronic or any type of laser), which comprises: a transition-metalion-containing solid state laser gain medium; a means for exciting saidlaser medium to emit coherent radiation said exciting means being apumping source comprising at least one laser diode operating at awavelength shorter than 800 nm; and an optical resonator means forgenerating and controlling said coherent radiation.

The solid state laser medium can be doped with various transition metalions including Cr⁺³. A preferred solid state laser medium is alexandritedoped with Cr⁺³. Useful laser diodes are visible semiconductor diodelasers based on stoichiometric compositions of AlGaInP.

The present invention also encompasses a method of pumping a laser(vibronic or otherwise) comprising the steps of: generating a laserdiode pumping beam at a wavelength shorter than 800 nm; exciting atransition-metal ion containing solid state laser gain medium (vibronicor otherwise) by impinging said laser diode pumping beam on saidtransition-metal ion-containing solid state laser gain medium, so as toexcite the laser medium; and an optical resonator to emit coherentradiation.

The laser medium can be alexandrite or emerald laser and the pumping isat a wavelength corresponding to the R line absorption and the lasing ison a vibronic or otherwise transition. Other laser media which can beused are Cr⁺³ :LiCaAlF₆ or Cr⁺³ :LiSrAlF₆ and with such media the diodelaser wavelength is shorter than 680 nm and the pumping is directly intothe broad absorption band. In the case where the laser medium is aTi:Al₂ O₃, the diode laser wavelength is shorter than 650 nm.

The laser diode pumped laser of the present invention can be employed inany possible laser applications, such as uses in spectroscopy,photochemistry, 25 communications and in medical procedures. In clinicalpractice, the present invention can be used in photodynamic therapy.Additionally, use can be made of the present laser diode pumped solidstate laser as an injecting source to control the frequency of a larger,more powerful, CW or pulsed tunable solid state laser. The solid statelaser can be frequency doubled or tripled by non-linear opticalprocesses to provide shorter wavelengths usually in the ultra-violet.

Many conventional tuning means may be used to control the laser of thisinvention. Examples of suitable tuning means include a prism, opticalgrating, birefringent filter, multilayer dielectric coated filters orlens having longitudinal chromatic aberration. Particularly suitable isa birefringent filter of the general type described by G. Holtom and O.Teschke, "Design of a Birefringent Filterefor High-Power Dye Lasers,"IEEE J. Wuantum Electron QE-10,577, 1974. This type of filter issometimes referred to as a type of "Lyot filter". (B. Lyot, Compt. Rend.197, 15983, 1933).

In operation, the laser of this invention is used to general tunablecoherent radiation. The process for operating the laser comprisesactivating the laser diode pumping means to excite the solid state lasermedium and optionally using a tuning meansfor example, rotating abirefringent plates to achieve the desired output wavelength. The laseroptionally includes cooling means for temperature control; i.e. tomaintain a desired temperature. For example, if the cooling meanscomprise a circulating fluid, the flow rate and temperature of the fluidcan be adjusted to maintain the desired temperature. The circulatingfluid may be air, water, a cryogenic liquid, etc. A heater may be usedto control the fluid temperature when operation at elevated temperaturesis desirable.

Optionally, the laser may include means for Q-switching. These means maycomprise a saturable dye absorber, an acousto-optic Q-switch or aPockels cell and polarizing element placed in the beam path. TheQ-switch "spoils" the Q of the optical resonator cavity for an intervalof time during which energy is stored. At the appropriate moment theQ-switch is turned to the low loss condition, and the stored energy inthe medium is suddenly released in a"giant pulse" of very shortduration. The laser may also be mode-locked using conventionalacoustooptic or electro-optic techniques. The laser output may befrequency doubled or tripled using non-linear optical materials placedinside or outside of the laser resonator.

The invention further encompasses an optical amplifier apparatuscomprising of: 1) a transition metalion containing solid state lasergain medium; 2) means for exciting said laser medium to emit radiation,said exciting means being a pumping source comprising at least one laserdiode operating at a wavelength shorter than 800 nm; and a means, suchas a dielectric or dichroic mirror, for introducing light at one or moreemitted wavelengths into the excited laser gain medium.

The apparatus includes wavelength control or tuning element such as aprism, grating, birefringent tuner, or etalon is inserted into theoptical resonator and an optical modulator, acousto-optic, orelectro-optic switch, such as a Pockels cell, can be inserted into theoptical resonator. A harmonic generator is inserted into the opticalresonator.

The laser diodes used in the invention can be visible semiconductorlaser diodes based on stoichiometric compositions of AlGaAs,stoichiometric compositions of GaN, or stoichiometric compositions ofZnSe.

The present apparatus can further comprise a heater, heat transferfluid, or other thermal control device to adjust or control thetemperature of the solid-state laser gain medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus r the pumping of atransition metal ion containing solid state laser by an exciting meanscomprising visible diode lasers.

FIG. 2 shows the broad band and R-line absorption spectra ofalexandrite.

FIG. 3 shows the broad band lasing spectrum of a diode pumpedalexandrite laser.

DETAILED DESCRIPTION OF THE DRAWING AND THE INVENTION

FIG. 1 illustrates a pumping apparatus according to the presentinvention. Two laser diodes (11, 12) provide pumping emission radiationwhich is collimated by means (13, 14) to form the collimated pumpingemission radiation 19, 20. Pumping emission radiation 19, 20 is combinedand aligned by means 15 and focused by focusing means 16 onto the lasingmedium 10. The lasing medium 10 can be provided with coated ends 21 and22 having conventional di-electric coatings. Coating 21 is on theexterior face of the lasing medium 10 and is a dichroic coating whichtransmits at the pumping wavelength and reflects the laser emission(18). Coating 22 is on an interior face of the medium and is ananti-reflection coating. The aligned pumping emission radiation iffocused by focusing means 16 onto the lasing medium 10 in order toexcite the lasing medium and to cause the emission of coherent laserradiation 18. An output coupler 17 which is partial reflector at thesolid state laser wavelength couples optical power out of the resonatorcavity.

The collimating (13, 14) aligning (15) and focusing means (16) arewell-known means to those skilled in the art. Possible such meansinclude lenses or mirrors or prisms. The free standing output couplermay be eliminated if the anti-reflection coatings on the gain medium isreplaced by a partially reflective, partially transmissive coating.Alternatively, both ends of the gain medium can be anti-reflectioncoated or cut at Brewster's angle and a free standing high reflector andoutput coupler utilized. This provides for convenient insertion ofoptical control elements (Q-switches, tuners etc.) tunes into thecavity.

A highly efficient and compact laser device can be achieved through thepumping of a transition metal ion doped solid state laser by one or morelaser diodes operating within the absorption spectrum of the transitionmetal ion. The laser diodes must supply sufficient energy which is atleast sufficient to exceed the lasing threshold of the lasing medium.The upper limit of the laser diode output is limited only by the natureof the semiconductor laser device.

Laser-active gain media useful in this invention include transitionmetal ion doped solid state crystals. The preferred lasing medium is achromium-doped beryllium aluminate (Cr⁺³ :Be Al₂ O₄) having thechrysoberyl structure ("alexandrite"). Other acceptable Cr⁺³ ioncontaining media include: emerald (Cr⁺³ :Be₃ Al₂ (Si; O₃)₆); Cr⁺³:SrAlF₅ ; Chromium GSGG (Gallium Scandium Gadolinium Garnet) or GSAG(Gallium Scandium Aluminum Garnet); chromium--LiCAF and LiSAF (Cr⁺³ :LiCaAlF₆ and Cr⁺³ :LiSrAlF₆); LGS (La₃ Ga₅ SiO₁₄); SCBO₃ ; and KZnF₃.The lasing medium could also be based on other divalent or trivalenttransition metal ions primarily doped into oxide or fluoride containinghosts such as Ti⁺³ :Sapphire (al₂ O₃); Co⁺² : MgF₂ ; V² :MgF⁺² ;Ce³ YLF;and Ni²⁺ MgO. Additionally, a Cr⁺⁴ vibronic laser material can be used.

The R-line spectral features of many Cr⁺³ materials, which areresponsible for 3 level lasing therein, provide sufficient absorptionfor longer wavelength pumping by the relatively low powered laser diodesso as to induce 4 level vibronic laser action. The absorption spectra ofalexandrite exhibits sharp R₁ and R₂ absorption features along thecrystallographic lo-axis at a wavelength of 680.4 nm. The spectra isalso characterized by a broad blue bank peak band peaking at 410 nmwhich consists of transition from the ground state to the ⁴ T₂ vibronicstate and a broad red band at 590 nm consisting of transitions to the ⁴T₂ state. The vibronic emission can therefore be optically pumped at thewavelength of the R line absorption or at any shorter wavelength. InCr⁺³ hosts, with low crystal field R-line absorption is absent andpumping is directly into the ⁴ T state.

Alexandrite (with Cr⁺³ concentrations in the range of 0.1-1 atomic %)can be made to lase by pumping with an exciting means comprising visiblelaser diodes, such as AlGaAs/GaAs or AlGaInP diode lasers, at awavelength equivalent to the R line wavelength of 680.4 nm. Lasing willoccur at a diode laser output of only 10 mw and may occur at even lowerpower levels. A semiconductor diode laser includes a single elementdiode or any array (linear or two dimensional in character) of diodelaser elements. The diode laser may be on continuously (cw pumping) orpulsed. The pumped laser may be of any configuration and may be cw,pulsed, Q-switched, mode-locked, or injection locked or seeded.

Pumping geometries useful in the present invention may be where thediode laser pump beam is colinear with the axis of lasing in the solidstate gain material (e.g., end pumped), transverse to the solid stategain material (side pumped) or at any angle in between. The shape to thegain media may be cylindrical (rod), prismodial (slab, nonplanar ring),or otherwise.

The coherent radiation emitted from the solid state lasing medium as aresult of pumping by the semiconductor laser beam is amplified in any ofseveral optical resonator configurations well known in the art. Apossible resonator is defined by two mirrors, one having hightransmission at the pumping beam wavelength and high reflectance at thelaser wavelength. The second mirror is partially transmissive (0.1% orgreater ) at the solids 7 state laser wavelength.

Example

A rod of alexandrite crystal (Cr⁺³ :BeAl₂ O₄) was placed in ahemispherical cavity and longitudinally pumped by two laser diodes.

The interior face of the lasing medium was anti-reflection coated from700 to 800 nm and the exterior face had a dichroic coating which wasgreater than 99.9% reflective from 757 to 773 nm and highly transmissiveat 680 nm.

Two 5 mW Sony AlGaInP laser diodes were used as the exciting means andwere operated at 675 and 677 nm at 25° C. It was therefore necessary totemperature tune their output to overlap the R1 line. It was found thatthe diodes 20 tuned on the average 0.25 nm/°C. and thus both wereoperated near 40° C. The threshold current for the lasers at thisoperating temperature was 82 mA and the power slope efficiency was 0.54mW/mW. The slope efficiency improved as the temperature increased, andwas 0.40 and 0.47 mW/mW at 15 and 25° C., respectively. The diodes couldbe overdriven for short periods of time to produce power in excess oftheir maximum ratings, a feature that turned out to be necessary toexceed pump threshold. The far-field emission was measured for one ofthe laser diodes operating at 3 mW and showed full width at half maximum(FWHM) divergence angles of 15° and 28° in the planes parallel andperpendicular to the junction, respectively. The spectral width of thislaser diode was measured to be 0.75 nm at 680 nm and contained severallongitudinal modes. The focused spot size of the laser diode on thealexandrite crystal was circular and measured 29 μm in diameter.Exceeding the maximum rated drive current by several mA for each laserdiode, sufficient power was provided to the alexandrite crystal to reachthreshold. The broadband lasing spectrum of alexandrite pumped by thelaser diodes is shown in FIG. 2.

Although only one embodiment of the present invention has been describedin detail, it should be understood that the present invention may beembodied in many other specific forms without departing from the spiritor scope of the invention.

What is claimed:
 1. An apparatus for generating coherent radiation whichcomprises:a transition-metal ion-containing solid state laser gainmedium; a pumping source comprising at least one laser diode operatingin the visible range for exciting said laser gain medium to emitradiation; and an optical resonator for generating coherent radiation.2. The apparatus of claim 1 wherein said solid state laser gain mediumis doped with Cr⁺³ ions.
 3. The apparatus of claim 1 wherein said solidstate laser gain medium is alexandrite.
 4. The apparatus of claim 1wherein said solid state laser gain medium is emerald Cr⁺³ :LiSrAlF₆ orCr⁺³ :LiCaAlF₆.
 5. The apparatus of claim 1 wherein said laser diodesare visible semiconductor diode lasers based on stoichiometriccompositions of AlGaInP.
 6. The apparatus of claim 1 wherein said lasergain medium is an alexandrite or emerald laser and the pumping is at awavelength corresponding to the R line absorption and the lasing is on avibronic transition.
 7. The apparatus of claim 1 wherein the solid statelaser gain medium is emerald Cr⁺³ :LiCaAlF₆ or Cr³ :LiSrAlF₆ and thediode laser wavelength is shorter than 680 nm.
 8. The apparatus of claim1 wherein the solid state laser gain medium is a Ti:Al₂ O₃ and the diodelaser wavelength is shorter than 650 nm.
 9. A method of generatingcoherent radiation, comprising the steps of:causing a laser diode togenerate pumping beam at a visible wavelength; exciting atransition-metal ion-containing solid state laser gain medium byimpinging said laser diode pumping beam on said transition-metalion-containing solid state laser medium, so as to excite the lasermedium to emit stimulated emission; and recirculating said stimulatedemission in an optical resonator.
 10. The method of claim 9 wherein saidsolid state laser gain medium is doped with Cr⁺³ ions.
 11. The method ofclaim 9 wherein said solid state laser gain medium is alexandrite. 12.The method of claim 9 wherein said solid state laser gain medium isemerald, Cr⁺³ :LiSrAlF₆ or Cr⁺³ :LiCaAlF₆.
 13. The method of claim 9wherein said laser diodes are semiconductor diode lasers based on astoichiometric compositions of AlGaInP.
 14. The method of claim 9wherein said solid state laser gain medium is an alexandrite or emeraldlaser and the laser diode pumping beam is generated at a wavelengthcorresponding to the R line absorption.
 15. The method of claim 9wherein said solid state laser gain medium emerald Cr⁺³ :LiCaAlF₆ orCr⁺³ :LiSrAlF₆ and the laser diode pumping beam is generated at awavelength shorter than 680 nm.
 16. The method of claim 9 wherein saidsolid state laser gain medium is Ti:Al₂ O₃ and the laser diode pumpingbeam is generated at a wavelength shorter than 650 nm.
 17. The apparatusof claim 1, wherein a wavelength control or tuning element is insertedinto the optical resonator.
 18. The apparatus of claim 1 wherein anoptical modulator acousto-optic, or electro-optic switch, is insertedinto the optical resonator.
 19. The apparatus of claim 1 wherein aharmonic generator is inserted into the optical resonator.
 20. Theapparatus of claim 1 wherein said laser diodes are visible semiconductorlaser diodes based on stoichiometric compositions of AlGaAs.
 21. Theapparatus of claim 1 wherein said laser diodes are visible semiconductorlaser diodes based on stoichiometric compositions of GaN.
 22. Theapparatus of claim 1 wherein said laser diodes are visible semiconductorlaser diodes based on stoichiometric compositions of ZnSe.
 23. Theapparatus of claim 1 wherein a heater, heat transfer fluid, or otherthermal control device is used to adjust or control the temperature ofthe solid-state laser gain medium.